1. Field of the Invention
The present invention relates to a gas-barrier, stretched polyamide film resistant to boiling treatment and retort treatment, which is excellent in flexibility, impact resistance, pin-hole resistance, and transparency.
2. Description of the Prior Art
Multi-layered films having a gas-barrier layer made of poly(vinylidene chloride) (PVDC), ethylene-vinyl alcohol copolymer (EVOH), polyamide, etc. have been used as gas-barrier packaging materials. Of various types of polyamides, a m-xylylene group-containing polyamide obtainable by the polycondensation of m-xylylenediamine and a C6-12 α,ω-aliphatic dicarboxylic acid, as compared with other types of gas-barrier resins, is characterized by a little reduction in the gas-barrier properties and a quick recovery of the gas-barrier properties upon boiling treatment and retort treatment. Particularly, poly(m-xylylene adipamide) obtainable using adipic acid as the C6-12 α,ω-aliphatic dicarboxylic acid (hereinafter optionally referred to as “Nylon MXD6”) is excellent in such characteristics and now comes to be widely used in the field of packaging.
Although Nylon MXD6 is low in the impact resistance, flexibility and pin-hole resistance in the non-stretched state, these properties can be improved to some extent by stretching. However, a sufficient improvement was not obtained for Nylon MXD6 alone, and therefore, other techniques for a further improvement have been demanded. To meet such demand, proposed is the addition of an additive material to Nylon MXD6, for example, the addition of polyolefin (JP 5-77373A), the addition of a modified polyolefin (Japanese Patents 3021854 and 3021851), the addition of an ionomer (JP 5-193081A, JP 7-117198A and JP 7-276591A), the addition of an polyamide elastomer (JP 8-224844A and JP 8-165427A), and the addition of a styrene-butadiene copolymer (JP 2000-169603A). Also proposed is the addition of a modified polyester elastomer to Nylon MXD6 (WO 90/09409). The resultant composition is taught to be excellent in the flexibility, chemical resistance, oil resistance, and moldability and suitable for automotive parts and electric machine parts.
However, any of the proposed methods described above fails to provide a sufficient impact resistance, flexibility and pin-hole resistance. Also, the addition of an elastomer causes the reduction of the gas-barrier properties and transparency. Particularly, in the method of JP 2000-169603A adding a styrene-butadiene copolymer, the improvement in the impact resistance is still further demanded.